Liquid creme or lotion shampoo formulations are often opacified by deliberately incorporating "pearlescing" agents therein to achieve a cosmetically attractive pearl-like appearance known as pearlescence.
Not all useful opacifiers, however, are pearlescing agents. Several materials are known to be useful for creating pearlescence in cosmetic compositions, simulated pearls and lacquers. These materials are sometimes lustrous, silvery-white substances derived from naturally occurring and inorganic materials, such as comminuted fish scales, natural mineral mica, mercuric chloride and shellfish nacres, or are synthetic forms of mother-of-pearl.
Pearlescing agents frequently utilized in cosmetic compositions may be stearic acid and insoluble metal salts thereof, such as magnesium stearate or zinc stearate, glyceryl stearates, ethylene glycol mono- and di-stearates, polyethylene glycol distearate, glycol amidostearate, less soluble surfactants having a high cloud point, such as (C.sub.16 -C.sub.18) alkyl glyceryl ether sulfonates, certain fatty alkanolamides, and even insoluble resin latex dispersions.
The type of pearlescence achieved may vary from flat to highly reflective or iridescent depending on the amount, size, shape and reflective or refractive power of the pearlescing agent used as the opacifier.
The known pearlescent materials suffer from several disadvantages. For example, non-separating, stable pearlescent compositions that contain a relatively large amount of water, such as shampoos, have been difficult to prepare. That is, once prepared, such compositions tend to separate during storage into two distinct phases, one of which contains a large amount of the pearlescent material.
Persons skilled in the art are aware of the problems faced by shampoo manufacturers in consistently preparing a pearly or pearlescent shampoo. A detailed discussion of these problems is found in the article "Opacifiers and pearling agents in shampoos" by Hunting, Cosmetics & Toiletries, Vol. 96, (hereinafter "Hunting") 65-78 (July, 1981), incorporated herein by reference. A particular problem, as described by Hunting, is the influence of the method or procedure used in preparing the shampoo, as well as the order in which the ingredients are incorporated.
Cosmetic compositions that contain ethylene glycol monostearate or distearate as a pearlescing agent can be made relatively stable to phase separation at ambient room temperatures. However, when the temperature of the composition is elevated near, to, or above the melting point of the pearlescer during summer storage or in a window display of a product, the composition must be cooled with agitation to return it to a pearlescent condition.
Another disadvantage of these compositions is that a generous amount of suspended pearl producing opacifier is required to produce pearlescence. Ethylene glycol monostearate or distearate, for example, are each typically utilized in amounts of about 1.5 to about 3.5 weight percent of the shampoo composition. When so used, those pearlescers also tend to interfere with lathering.
A disadvantage of some of the known particulate pearlescent materials such as comminuted fish scales, mica or shellfish nacre is that these pearlescers can be abrasive when used in a shampoo that is intended to be rubbed on the scalp and hair.
On the other hand, non-particulate, non-abrasive, wax-like pearlescers depend on their being suspended in the liquid medium in a crystalline state to produce pearlescence. These pearlescers, however, have several disadvantages. The compositions must frequently be stored and aged over a period of several weeks or more before the pearlescer reaches a satisfactory crystalline state. Such aging storage is commercially impractical, since the quality and consistency of the result is not readily predictable. Additionally, the viscosity of shampoos prepared with such pearlescers tends to continuously change to some unpredictable level after pearlescence initially develops, thereby presenting added product quality control problems.
In my U.S. Pat. No. 4,438,096, issued Mar. 20, 1984, to the assignee of this invention, I teach the use of the fatty acid ester, myristyl myristate, as the pearlescing agent for a shampoo prepared by a direct-type method disclosed therein. The teachings of that patent are incorporated herein by reference, and the method of preparing a shampoo of that patent is also referrred to as the "Preston" method.
In the Preston method, the myristyl myristate was admixed directly with the liquid medium of the composition, while the ester was in a liquid state and the liquid medium was heated to a temperature above the melting point of myristyl myristate. The liquid medium contained water and an amount of surface active agent effective for cleansing.
Myristyl myristate was noted in my above patent as being a singularly useful pearlescing agent in the group of fatty alcohol esters of fatty acids that are homologous esters of myristyl myristate for shampoos prepared according to the method taught therein. For example, cetyl palmitate did not form a pearlescent shampoo when it was substituted for myristyl myristate.
My patent also teaches that at low levels of about 0.2 weight percent of myristyl myristate, pearlescence developed slowly over an aging period of about two weeks, and that in amounts greater than about 1 weight percent of the shampoo, phase separation of the fatty acid ester from the rest of the shampoo took place. Thus, where the total amount of pearlescing agent used fell outside the range of about 0.25 to about 1 weight percent of the total composition, the addition of phase-stabilizing ingredients was needed for preparation of stable, non-separating pearlescent compositions.
There is a need, therefore, for a pearlescing agent and method of preparing a pearlescent product capable of producing a consistently predictable pearlescence, especially in a shampoo, within a commercially reasonable time period.